(1) Field of the Invention
The present invention relates to a system and a method for determining the position of a moving object and more specifically to a system and a method for determining the position of an elevator car.
(2) Prior Art
A technique, known as the PVT position approximation technique, has been widely used in industry to determine the position of elevator cars. The PVT technique uses machine encoder information, also known as the primary velocity transducer or PVT, corrected to vanes mounted at fixed locations in the hoistway. Determining car position in express zones presents a particular challenge since a PVT-based approximation system may have errors due to rope stretch, slip, etc. The car position may be corrected upon detection of a door zone vane at the end of the express zone; however, the longer the express zone the more difficult it is to blend in the PVT-based position feedback with the vane-based position feedback. In order to provide a smoother transition, additional vanes have been mounted in the express zone, thus increasing the installed cost.
Elevator safety codes require that traction elevators be provided with terminal stopping devices, such as a normal terminal stopping device (NTSD), an emergency terminal speed limiting device (ETSLD), an emergency terminal stopping device (ETSD), and final terminal stopping devices. ETSLD is used on elevators with reduced stroke buffer, while ETSD is used on elevators with full stroke buffer. These devices use car position and speed information near the top and bottom of the hoistway to (1) bring the car to a controlled slowdown and stop at or near the terminal landing (NTSD), or (2) generate an emergency stop by removing power from the driving machine and brake (ETSD and ETSLD and final terminal stopping devices).
Codes also require independence between the normal control system, NTSD, and ETSD, as summarized below. Operation of ETSLD must be entirely independent of the operation of NTSD. The car speed sensing device for ETSLD must be independent of the normal speed control system. ETSD must function independent of the NTSD and of the normal speed control system.
The main disadvantage of current systems is the relatively high installed cost resulting from the multitude of sensors and vanes, mounted on different tracks (for NTSD, ETSD and door zones) and an additional channel on machine speed encoder.